1. Field of the Invention
The present invention relates to a method for forming a functional element using vanadium dioxide (VO2) as a metal-to-insulator transition material, a functional element formed by the method, a method for producing a functional device, and a functional device produced by the method.
2. Description of the Related Art
Hitherto, wiring for use in semiconductor devices including active elements and passive elements has been composed of a metal such as aluminum or copper. A wiring pattern is formed by a step of forming a metal film by, for example, sputtering or evaporation of a metal, a step of forming a resist layer, a step of exposing a mask, a developing step, a step of etching a metal film, a step of removing the resist layer, and so forth. In the case where a wiring pattern is not formed by a single layer, the wiring pattern is separated into a plurality of wiring layers. The wiring layers are electrically connected through via plugs provided between the layers.
For example, Japanese Unexamined Patent Application Publication No. 2002-158418 (see paragraph Nos. 0005 to 0008 and 0015 to 0018, and FIG. 1) entitled “Wiring board and method for producing same” discloses a technique in which a wiring pattern is formed without using an etching step and the wiring pattern can be changed and modified even after the formation of the wiring pattern. The following is described in this patent document.
In a wiring board including wiring between elements (for example, electronic or electric circuits), the wiring is formed of a conductive state of a photo-induced phase transition material that can exist in an insulating state, which is a stable state, and the conductive state, which is a metastable state.
Regarding a method for producing the wiring board, in a step of forming a conductive state of the photo-induced phase transition material by light, the wiring pattern is formed with a mask or is directly formed by laser scanning irradiation.
The material used for the wiring is mainly composed of a transition metal or an oxide thereof that undergoes a phase transition by light irradiation or heat application. Typical examples of such a transition metal oxide include compounds represented by the formula AxByOz (wherein x, y, and z are in the ranges of 1≦x≦14, 1≦y≦24, and 1≦z≦41; and A and B may be the same or different elements). In the transition metal oxide, energy states of atoms seem to be changed by light irradiation to cause the displacement of a crystalline lattice, so that a phase transition phenomenon occurs. These are described in Japanese Unexamined Patent Application Publication No. 2002-158418.
An example of a material exhibiting a metal-to-insulator transition is vanadium dioxide (VO2). VO2 is an oxide which has a monoclinic structure at room temperature and undergoes the metal-to-insulator transition at about 68° C. to a rutile structure. The electrical resistivity of VO2 changes three orders of magnitude or more. VO2 is used for an infrared bolometer-type temperature sensor because of a high rate of change in electrical resistivity with temperature (for example, see Japanese Unexamined Patent Application Publication No. 2007-225532 (paragraph No. 0036 to 0041, and FIG. 1).
Vanadium dioxide has a different crystal phase, VO2 (B), which does not exhibit the metal-to-insulator transition. Thus, vanadium dioxide that exhibits the monoclinic-to-rutile transition is usually expressed as VO2 (M) or VO2 (R).
A thin film of vanadium dioxide usually expressed as VO2 (M) or VO2 (R) is reported to be subjected to the metal-to-insulator transition by the application of an electric field. There is a possibility that vanadium dioxide can be applied to field-effect transistors, switching elements, and memory elements (for example, see PCT Japanese Translation Patent Publication No. 2006-526273 (paragraph Nos. 0025 to 0028, FIGS. 3 to 5), Japanese Unexamined Patent Application Publication No. 2007-224390 (paragraph Nos. 0026 to 0039 and 0077 to 0080, FIGS. 1 and 11 to 13), PCT Japanese Translation Patent Publication No. 2007-515055 (paragraph Nos. 0007 to 0022, FIGS. 1 and 2), Japanese Unexamined Patent Application Publication No. 2008-205140 (paragraph Nos. 0049 and 0058 to 0065, FIGS. 4 and 6 to 9), B. Guiton et al., “Single-Crystalline Vanadium Dioxide Nanowire with Rectangular Cross Sections”, JACS, 127, 498-499 (2005) (page 498, left column, lines 22 to 36, and page 499, right column, lines 12 to 15), H-T. Kim, et al. “Raman study of electro-field-induced first-order metal-to-insulator transition VO2-based devices”, Applied Physics Letters, 86, 242101 (2005) (242101-2, left column, FIG. 1).